The present invention relates to a method for the production of mineral fibers with uniform fiber length characteristics.
It is well known that the mineral fibers such as mineral wool, are produced, as a rule, by melting in a melting oven, preferably a cupola oven. The mineral forming the mineral fibers, e.g. a mixture of suitable stone species, kinds of glass or types of slag, is melted by means of heat obtained by combustion of some suitable material, usually coke. Thereafter, the melt is tapped off at the bottom part of the oven and is brought to flow down onto a fiber-forming aggregate. The fiber-forming aggregate includes, in the best systems for production of mineral fibers and mineral wool, a spinner wheel or a cascade of a plurality of spinner wheels. The melt is thrown out by the centrifugal force of the wheel, and is caught by a stream of air, which transfers the melt to a perforated collection band on which the fibers formed from the melt are deposited simultaneously as the motive air leaves through the perforations of the band.
In such a method, one will not get fibers of interminable length: the fibers are of limited length, which is, however, subject to great mutual variations. The fibers are not positioned in a straight line, because the conveyor band is moving at a much lower speed than the speed at which the fibers are thrown out. Consequently, the fibers are laid down in the form of loops or bends, and, in addition, a plurality of different fibers will become interwoven into each other.
Now, it has proved that one may advantageously use the mineral fibers as a re-inforcement means in different case materials, especially plastics, and that the re-inforcement power is, to a very high extent, dependent upon the fiber length.
It will be evident from the above, that it is important, firstly that one can measure the fiber length, and secondly that one can influence the production means in such a way that the desired fiber length will be obtained. It is not possible, in practice, to produce mineral fibers by economically defendable means, which possess a constant fiber length. Therefore, one has had to be satisfied with a given mean fiber length. However, even then, certain difficulties remain. It is obvious that with a given mean value of the fiber length, the mineral fibers may have very different properties dependent upon if there is a great difference between the shortest and the longest fiber length, or if this difference is only rather small. Thus, there are two different and important magnitudes for the fiber length: on the one hand, the mean fiber length; and on the other hand, the extent of the deviation from the mean fiber length. The greater the spreading of fiber length is at a given mean value, the more "long fibrous" will the mineral fiber appear, and this property of being long fibrous is an essential parameter in what has been called "the fiber length characteristic" of the mineral fiber. This characteristic, therefore could be expressed in the form of an expression having two variables, viz. L.+-.f(1). L indicates in this formula the mean fiber length and f(1) is a function indicating partly the number of fibers deviating from the mean fiber length and partly the amount of such deviation. The functional connection, thus, will be a little complicated.
It was previously known that measurement of the fiber length may under given circumstances be desirable. However, there has been no reliable and suitable method available for the execution of the measurement. As a matter of fact, the known measurement took place as follows: (1) a small quantity of mineral fibers is spread out into a very thin layer and optically reproduced on a screen, so that one could follow each separate fiber in its run; and, (2) with a so called map meter (an instrument comprising a small rifled pulley in connection with a measurement or counting device), the fiber length on the screen or table, on which the reproduction takes place, is measured. The measurement of a single fiber, in the said method, takes an unreasonably long time, and if it is desired to state the properties of a mineral wool comprising a great number of fibers by their fiber length characteristic, this method consumes such a long time, that the method is uneconomical. Moreover, and this is perhaps the more important thing, it is so tedious, that it cannot be used during a production run. On the other hand, it is obvious that if it is desired to produce mineral fiber material having a given fiber length characteristic, it must be possible quickly to correct the errors created due to the unavoidable variations in the behavior of the system. These variations depend upon certain relations known per se, but some may be used for controlling the behavior of the system, in order to get at least approximately the desired fiber length characteristic. If this is done, however, it becomes necessary to find a way sufficiently quickly to measure the variables contained in the fiber length characteristic and to cause them to influence the production system in order to improve the fiber length characteristic.
The relations in the production system, which influence the fiber length characteristic, firstly so that fiber length will be changed upon involuntary changes of the said relations, and secondly so that one may, by influencing said relations, correct existing deviations, include the following:
A first matter acting upon these relations is the choice of the raw material. The raw material is charged into the melting oven at its upper end. Before an amount of melt, determined by the choice of the raw material, has reached the bottom of the melting oven in order to be tapped off to a fiber-forming aggregate, usually rather a long time will lapse. Influencing the properties by choice of the raw material, of course, may be used in the sense that one may by long-time tests state that a given raw material will, as a rule, give a more favourable fiber length characteristic. But a quick control of deviations in the fiber length characteristic can scarcely be possible in practice in this way.
Similar conditions apply for the amount of fuel fed. The variations existing in the fiber length characteristic as a result of variation of the quantity of fuel fed also act rather long periodically, and therefore they cannot be used for quick control of the fiber length characteristics of the mineral fibers.
Amongst the quick acting means may be mentioned the type of blast air conpulsorily fed to the oven, which may have different temperature, different pressure, or different amount of oxygen. (The last mentioned provided that oxygen is added to the blast air in controlled amount.) Secondly the speed of the rotating spinner wheels may be mentioned. Both of these conditions may be controlled rather quickly, and especially the speed of the spinner wheels may be controlled at the highest possible speed, so that this type of control may be said to be practically instantaneous.
Of the two last mentioned control means, the type of the blast air has proved to be the one most easily available.
It will be obvious that the method according to the invention must comprise both the step of measuring the fiber length characteristic of the mineral fiber product by means of a quick acting measurement method, and also the step, by means of the result of the measurement, to correct deviations which may perhaps exist regarding the fiber length characteristics in relation to a pre-determined desired value. The greatest difficulty in solving this problem has been to find a measurement method for the fiber length characteristic, which is sufficiently quick to be used during a production run, and which is also sufficiently accurate.